1. Field of the Invention
This invention relates to a method of detecting an abnormal load acting on a servomotor which drives a cutter of a machine tool or a robot arm, and of controlling the servomotor when such abnormal load is detected.
2. Description of the Related Art
In a machine tool or a robot using a servomotor as a drive source, when a movable element of the machine tool such as a cutter or a table, or a robot arm collide with an obstacle, such a collision causes the servomotor to develop a large torque as it continues to drive the movable element of the machine tool or the robot arm in response to a given move command. Hence, the movable element of the machine tool or the robot arm is subjected to an abnormally large force which may damage the driving mechanism including the servomotor. To prevent such a damage, there is known a method including detecting an abnormal load acting on the servomotor, determining an occurrence of collision when an abnormal load is detected, and stopping the motor in such abnormal load condition. In the International Publication WO91/10181, there is proposed a method of estimating a disturbance torque using a disturbance estimating observer, and detecting an abnormal load by comparing the estimated disturbance torque with a predetermined set value.
According to the above conventional method, when an abnormal load is detected, the speed command is set to "0", thereby stopping the servomotor immediately.
However, when abnormal load is detected, the motor is already affected by the disturbance load. Since the machine has elastically deformable components, even when the motor has stopped immediately in response to the detection of the abnormal load, the motor causes the machine to be kept depressed or the machine to be kept elastically deformed, thereby causing the machine to be continuously subjected to the load and the resulting damages to the tools and machine components.